Page 107 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.3 Mitosis: Cell Division That Preserves Chromosome Number 99

become arrested in a resting form of G 1 known as G 0 and do molecules as long as 3–4 cm condense into discrete chro-
not normally divide again during a person’s lifetime. mosomes whose length is measured in microns (millionths
Synthesis (S) is the time when the cell duplicates its of a meter). This process is equivalent to compacting a 200 m
genetic material by synthesizing DNA. During duplication, length of thin string (as long as two football fields) into a
each chromosome doubles to produce identical sister chro- cylinder 8 mm long and 1 mm wide.
matids that will become visible when the chromosomes con- Another visible change in chromatin also takes place
dense at the beginning of mitosis. The two sister chromatids during prophase: The darkly staining nucleoli begin to break
remain joined to each other at their centromeres. (Note that down and disappear. As a result, the manufacture of ribo-
this joined structure is considered a single chromosome as somes ceases, providing one indication that general cellular
long as the connection between sister chromatids is main- metabolism shuts down so that the cell can focus its energy
tained.) The replication of chromosomes during S phase is on chromosome movements and cellular division.
crucial; the genetic material must be copied exactly so that Several important events that characterize prophase oc-
both daughter cells receive identical sets of chromosomes. cur outside the nucleus in the cytoplasm. The centrosomes,
Gap 2 (G 2 ) is the interval between chromosome dupli- which replicated during interphase, now move apart and be-
cation and the beginning of mitosis. During this time, the come clearly distinguishable as two separate entities in the
cell may grow (usually less than during G 1 ); it also synthe- light microscope. At the same time, the interphase scaffold-
sizes proteins that are essential to the subsequent steps of ing of long, stable microtubules disappears and is replaced
mitosis itself. by a set of dynamic microtubules that rapidly grow from and
In addition, during interphase an array of fine microtu- shrink back toward their centrosomal organizing centers.
bules crucial for many biochemical processes becomes visi- The centrosomes continue to move apart, migrating around
ble outside the nucleus. The microtubules radiate out into the the nuclear envelope toward opposite ends of the nucleus,
cytoplasm from a single organizing center known as the apparently propelled by forces exerted between interdigi-
centrosome, usually located near the nuclear envelope. In tated microtubules extending from the two centrosomes.
animal cells, the discernible core of each centrosome is a
pair of small, darkly staining bodies called centrioles
(Fig. 4.10a); the microtubule-organizing center of plants Prometaphase: The spindle forms (Fig. 4.10b)
does not contain centrioles. During the S and G 2 stages of Prometaphase (before middle stage) begins with the break-
interphase, the centrosomes replicate, producing two cen- down of the nuclear envelope, which allows microtubules ex-
trosomes that remain in extremely close proximity. tending from the two centrosomes to invade the nucleus.
Chromosomes attach to these microtubules through the
kinetochore, a structure in the centromere region of each
During Mitosis, Sister Chromatids chromatid that is specialized for conveyance. Each kineto-
Separate and Two Daughter Nuclei Form chore contains proteins that act as molecular motors, enabling

Although the rigorously choreographed events of nuclear the chromosome to slide along the microtubule. When the ki-
and cellular division occur as a dynamic and continuous netochore of a chromatid originally contacts a microtubule at
process, scientists traditionally analyze the process in sepa- prometaphase, the kinetochore-based motor moves the entire
rate stages marked by visible cytological events. The art- chromosome toward the centrosome from which that microtu-
ist’s sketches in Fig. 4.10 illustrate these stages in the bule radiates. Microtubules growing from the two centro-
nematode Ascaris, whose diploid cells contain only four somes capture chromosomes by connecting first to the
chromosomes (two pairs of homologous chromosomes). kinetochore of one of the two sister chromatids, chosen at
random. As a result, it is sometimes possible to observe groups
of chromosomes congregating in the vicinity of each centro-
Prophase: Chromosomes condense (Fig. 4.10a) some. In this early part of prometaphase, for each chromo-
During all of interphase, the cell nucleus remains intact, some, one chromatid’s kinetochore is attached to a microtubule,
and the chromosomes are indistinguishable aggregates of but the sister chromatid’s kinetochore remains unattached.
chromatin. At prophase (from the Greek pro- meaning During prometaphase, three different types of microtu-
before), the gradual emergence, or condensation, of indi- bule fibers together form the mitotic spindle. All of these
vidual chromosomes from the undifferentiated mass of microtubule classes originate from the centrosomes, which
chromatin marks the beginning of mitosis. Each condens- function as the two poles of the spindle apparatus. Microtu-
ing chromosome has already been duplicated during inter- bules that extend between a centrosome and the kinetochore
phase and thus consists of sister chromatids attached at of a chromatid are called kinetochore microtubules, or cen-
their centromeres. At this stage in Ascaris cells, therefore, tromeric fibers. Microtubules from each centrosome that are
four chromosomes exist with a total of eight chromatids. directed toward the middle of the cell are polar microtu-
The progressive appearance of an array of individual bules; polar microtubules originating in opposite centro-
chromosomes is a truly impressive event. Interphase DNA somes interdigitate near the cell’s equator. Finally, short